The present invention relates to a process for conversion of gaseous UF.sub.6 to UO.sub.2 powders which is suitable for production of nuclear fuels in power plant owing to its good ceramic properties, small fluorine contents and free flowability.
As a process for converting UF.sub.6 into UO.sub.2 powders for nuclear fuels in a power plant, gaseous UF.sub.6 has been conventionally converted to be UO.sub.2 powder in a industrial scale by two different process, that is, a wet process and a dry process. The wet process is defective in that many steps required make the operation complex and a large quantity of waste solution is produced.
On the other hand, the dry process has defects of having poor ceramic properties of UO.sub.2 powders as a product and a large fluorine contents of the product, but it has advantages of having simple steps and also a small quantity of waste solution produced. Therefore, adoption of the dry process has been recently increased by overcoming the defects above mentioned. As the dry process, there are a process using a rotary kiln, a process using a fluidized bed reaction apparatus and a process using a flame combustion reaction apparatus. Of these processes, the process using a fluidized bed reaction apparatus produces UO powders as a product which has a free flowability, thus making handling of the UO.sub.2 powders in following steps very much easier, as compared with that of the other processes.
In the process using a fluidized bed reaction apparatus which has the advantages mentioned, the ceramic properties of the UO.sub.2 powders produced become poorer and also the fluorine contents thereof become larger, as compared with those of the other processes. The poor ceramic properties of the UO.sub.2 powders is due to a formation of UO.sub.2 F.sub.2 in fine particles by gas phase reaction of gaseous UF.sub.6 with steam as shown in the following equation (1) and a formation of UF.sub.4 in converting of UO.sub.2 F.sub.2 to UO.sub.2 with hydrogen gas as shown in the following equations (2) and (3). In the conventional dry processes, especially in the process using a fluidized bed reaction apparatus, the reaction is mostly composed of the following two stage reactions. EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (1) EQU UO.sub.2 F.sub.2 +H.sub.2 .fwdarw.UO.sub.2 +2HF (2)
In this process, UF.sub.4 is apt to be formed by a reverse reaction as shown in the equation (3). Namely, the UO powder may possibly be hydrofluorinated to UF powder. EQU UO.sub.2 +4HF.fwdarw.UF.sub.4 +2H.sub.2 O (3)
UF.sub.4 is a substance which is apt to sinter at a relatively low temperature (about 1000.degree. C.) and begins to sinter at the operating temperature of the equation (2) to hinder a defluorinating reaction which is important for lowering fluorine contents of UO.sub.2 powder as a product. Therefore, it was formerly required to add an excess of steam in the equation (2) to suppress hydrofluorination of the UO.sub.2 powder. As a result, the fluidized bed operation became more complex and at the same time the excessively added steam increased a quantity of waste solution substantially. Further, as a long time was required for defluorinating UO.sub.2 powder as a product, it was exposed to a high temperature for a long time. Consequently the ceramic properties of UO.sub.2 powders were apt to be substantially reduced.
Further, another defect of the case in which the fluidized bed reaction apparatus is used relates to a stability of the operation of the fluidized bed which converts UF.sub.6 into UO.sub.2 F.sub.2.
Namely, the UO.sub.2 F.sub.2 particles form the fluidized bed, but gaseous UF.sub.6 blown into the fluidized bed reacts with steam as a fluidizing gas introduced through the bottom of the fluidized bed to form UO.sub.2 F.sub.2 particles which deposit on the surface of UO.sub.2 F.sub.2 particles already existing. As the result, the thus deposited UO.sub.2 F.sub.2 cause growth of the UO.sub.2 F.sub.2 particles. On the other hand, a part of the UO.sub.2 F.sub.2 particles are pulverized by abrasion owing to collison with each other. The mean particle size of the UO.sub.2 F.sub.2 particles is determined by these balances, but in the conventional fluidized bed reaction apparatus, the thus obtained UO.sub.2 F.sub.2 particles are apt to grow substantially. Therefore, it was required to supply new UO.sub.2 F.sub.2 particles into the fluidized bed to maintain a stable fluidized bed operation. Consequently, the process system became complicated and the operation also became troublesome.